Such a network is comprised of a set of devices interconnected by a digital bus, for example a bus according to the standard IEEE 1394. It particularly comprises two types of devices:                Source devices capable of sending data over the network: These devices can recover the data through a “channel” external to the network.        Presentation devices adapted to receive the data circulating on the network, to process it or present it to the user.        
Hence, if the example of a digital home network designed to carry audio and/or video data to the various rooms of a house is used, the source devices are for example digital decoders receiving video programs from outside the network via a satellite antenna or via a cable connection, or even optical disc drives broadcasting data (audio and/or video) in digital form, on the network, read from a disc (in this case, the disc contains data coming from outside the network). The presentation devices are for example television receivers that can display video programs received from the network or, more generally, any type of device with the capability of decrypting encrypted data.
If one considers the viewpoint of the content providers that supply data coming from outside the local network, particularly from service providers broadcasting Pay TV programs or even optical disc editors for example, it is necessary to ensure that this transmitted data cannot be copied and can freely circulate (for example by being copied onto an optical disc or any other recording support) from one local network to another.
For this, it is known that data can be transmitted in secret form by encrypting it with cryptography algorithms using keys that are known beforehand by the devices authorised to receive this data or else that are exchanged according to specific secure protocols between the content provider and these devices.
The patent application PCT WO 00/62505 in the name of THOMSON multimedia, filed on Mar. 31, 2000 and claiming the priority of a French patent application in the name of the same applicant, filed on Apr. 13, 1999 and published under the reference FR 2792482, relates to a home network in which a public key specific to the network is used to encrypt the data circulating between the devices of the network, typically from the source devices mentioned above toward presentation devices. Only the presentation devices of this network have the private key corresponding to the public key. The pair (public key, private key) being specific to the network, data encrypted within the framework of this network cannot be decrypted by the devices of another network.
The use of a pair of asymmetric keys has some advantages, but also a few disadvantages. One of the main advantages is that no secret is memorized in the source devices: these devices know the public key but not the private key. However, the implementation of asymmetric keys is relatively slow with respect to that of symmetric keys. Moreover, the lifetime of asymmetric keys is short, demanding a regular revocation and the creation of new keys. In this case, data encrypted with a key, then recorded, can suddenly no longer be decrypted on the network. In addition, a large number of asymmetric keys are required.
The use of a symmetric key to encrypt the data would be considered as attractive. However, this would require the source devices to know this key, which would impose increased security constraints on them and consequently make them more expensive.
The present invention aims to solve the above-mentioned problems.